Electronic Article Surveillance (“EAS”) systems are designed to prevent unauthorized removal of an item from a controlled area. For example, EAS systems are often implemented at retail sales locations to deter theft and notify authorized personnel when shoplifting occurs. A typical EAS system may include a monitoring system and one or more security tags. The monitoring system may create an interrogation zone at an access point for the controlled area, e.g., at entry/exit doors in a retail store. A security tag may be fastened to an item, such as an article of clothing. If an active tag then enters the interrogation zone, an alarm may be triggered indicating unauthorized removal of the tagged item from the controlled area.
In many environments, there are a number of different EAS systems implemented simultaneously. Examples of such environments include small stores arranged, for example, in a conventional mall, strip mall or shopping plaza. As shown in FIG. 1, each EAS system generally operates by alternating periods of transmission, reception and idle or “sleep” time where the EAS system is not attempting to detect security tags, but may perform various processing or operational functions. In one known system, the EAS system operates at a frequency of 1.5 times the power line frequency, e.g., 90 Hz for a 60 Hz line frequency or 75 Hz for a 50 Hz line frequency and timing the beginning of transmit or receive windows with the zero-crossing point of the power line. During a “transmit” window, the EAS system does not receive and vice versa. However, the detection capability of an EAS system can be greatly reduced due to interference signals created by other nearby EAS systems having an “out of phase” transmitter operating during the “receive” window.
Historically, EAS transmitters in close proximity to each other have been synchronized to avoid these adverse interactions. This compatibility has been accomplished using several different levels of synchronization. For example, the carrier oscillators or the modulating waveform of transmitters can be synchronized. In more complex systems, such as those sold by Sensormatic Electronics Corporation under the trademark ULTRA*MAX®, a transmitter configuration sequence may be synchronized between multiple systems.
U.S. Pat. No. 6,201,469, for example, provides for synchronization of the transmitter configuration sequence using a power line zero crossing function for which the phase is manually adjusted, the entire contents of which are hereby incorporated by reference. U.S. Pat. No. 7,212,117 provides for a wireless phase locked loop (“PLL”) system for synchronizing the transmit carrier's modulating waveform, the entire contents of which are hereby incorporated by reference. U.S. patent application Ser. No. 11/729,372 provides a system for synchronization that utilizes a synchronization master signal that is generated from a global positioning satellite reference signal, the entire contents of which are hereby incorporated by reference.
Without this synchronization, EAS systems positioned within a certain proximity of one another may interfere with one another's receivers, thereby decreasing sensitivity, causing false alarms, or even rendering the system inoperable. This interference may, in turn, result in service calls to local technicians. The technicians then have to come to the site of the installed system and manually adjust the timing of the systems. A persistent or repetitive problem results in many duplicative service calls causing great expense and aggravation. Additionally, the interfering system may be inaccessible to the service personnel, thus it may not even be possible to synchronize the interfering system.
Further enhancing the problem, not all EAS systems available today utilize synchronization. The issue is even further complicated in that some unsynchronized EAS systems also utilize the random transmission of pulses in a non-periodic manner. One such system is described in U.S. Pat. No. 6,750,768. Use of an unsynchronized EAS system further increases the probability that these systems may interfere with each other and with synchronized systems as more and more of such systems are operated in close proximity with each other.
Therefore, what is needed is a system and method for reducing interference among close proximity EAS transmitters without the need for synchronizing between the individual transmitters.